The present disclose relates to display devices, and to illumination units.
Display devices are known that include a transmissive image display unit by which the transmittance of the incident light falling on the back side is controlled for image display. Display devices including a reflective image display unit that controls the reflectance of external light for image display are also known. A transmissive liquid crystal display panel is an example of such a transmissive image display unit. Examples of the reflective image display unit include a reflective liquid crystal display panel and electronic paper.
The reflective liquid crystal display panel includes a reflecting film that reflects external light, and displays an image by controlling the reflectance of external light with a liquid crystal layer. The electronic paper displays an image by varying the surface reflectance, which is achieved by, for example, moving the white and black pigments according to image patterns. Display devices provided with a reflective image display unit can use external light for image display. The display device with a reflective image display unit can thus realize low power consumption, thinness, and lightness, and have use as, for example, a portable device.
Display devices provided with a transmissive image display unit typically includes a backlight that shines light on the back side of the image display unit. The display device with a transmissive image display unit can control and provide image visibility by increasing the intensity of the backlight in high ambient illuminance, and decreasing the intensity of the backlight in low ambient illuminance. For example, JP-A-6-27440 describes a display device that controls the backlight luminance, which is raised to a reference luminance under bright external light illuminance conditions, and lowered under dark external light illuminance conditions.
In the display device with a reflective image display unit, desirable images can be recognized, for example, when the ambient illuminance can sufficiently provide photopic vision. However, image visibility suffers when the ambient illuminance can only provide mesopic vision or scotopic vision. As a countermeasure, a display device is proposed that includes an illumination unit that shines light on the front side of the reflective image display unit to increase the image luminance and improve visibility in a low illuminance environment.